Domains of Application—
Radio frequency (RF) waves in the microwave (uW), millimeter-wave (MMW) and sub-MMW (THz) bands are used in communications applications, imaging applications, and other measurement applications. Many of these applications would benefit from an efficient spatial modulator.
For imaging applications, THz beams penetrate most dielectric materials and non-polar liquids, permitting new imaging applications that are unachievable at other frequencies. THz wavelengths enable imaging capabilities with millimeter-scale resolution, as well as inexpensive and compact beam focusing. Applications of known importance include real-time low noise active imaging for covert rotorcraft navigation and landing in brownout conditions. Space applications include detection, imaging, and tracking of non-metallic debris and sun-masked objects. Due to its harmless interaction with living tissues, THz imaging provides new solutions for standoff imaging with application to non-destructive testing, as well as detection of contraband devices, illegal drugs, and explosive materials in military, homeland security, or correctional institution settings. THz frequencies are very attractive for applications where ionizing radiation is not tolerated. THz imaging can be used to detect materials hidden in clothing, and could be used to screen for shop-lifted items or other stolen goods. THz imaging can be used to view the contents of containers without opening them. THz imaging may also be useful for nondestructive noncontact subsurface inspection of structures, including composites.
For communication applications, THz beams offer very large absolute bandwidths at a region of the electromagnetic spectrum that is currently under-utilized throughout the world, and available for new communication systems. THz beams are relatively-difficult to create and modulate, owing to the high frequencies and lack of efficient active electronic devices at those frequencies. Spatial modulators would allow steering and information coding on THz beams.
Dynamic RF Diffractive Spatial Modulators—
Prior art in this area teaches spatial modulation of the RF phase front of an incoming wave using wavelength-scale zoned diffractive patterns, with reflective or absorptive regions having dimensions on the order of the RF wavelength. Prior art specifically teaches the use of Fresnel Zone Plate diffractive patterns. For example, Koolish, U.S. Pat. No. 6,720,936 B1, also teaches a derivative of the Fresnel Zone Plate, the Photon Sieve diffractive pattern. These prior-art wavelength-scale diffractive spatial modulators can be designed using only scalar diffraction theory (Kirchoff diffraction theory, Fresnel-Kirchoff diffraction theory) as typified in the well-known Fresnel Zone Plate equations.
Spatial modulation in prior art dynamic RF diffractive spatial modulators is effectively achieved through either phase modulation or amplitude modulation, where spatial regions of the Fresnel Zone Plate pattern absorb, block, or re-direct portions of the incoming beam, causing the remaining portions to have altered propagation (steered, focused, etc.). Because energy that is absorbed, blocked, or re-directed cannot be diffracted into the desired output beam, amplitude-modulating zone plates have lower diffractive efficiency compared to phase-modulating zone plates. It is also understood that spatial modulators which combine amplitude and phase modulation effects are possible.
Generation of Plasma to Create High-Loss or High-Reflectivity Regions in a Semiconductor:
Prior art discloses methods and apparatus that can be used to dynamically spatially-modulate RF beams using spatially-patterned volumes of high electronic carrier density (electron-hole plasmas). Specifically, this prior art discloses the creation of diffractive Fresnel Zone Plates (FZPs) via spatial patterning of carrier-dense plasma zones in a semiconductor. The carrier-dense plasma zones can be generated optically, with photons converted into electron-hole pairs in the semiconductor. The carrier-dense plasma zones can also be generated via direct injection of electrons via contacts, as discussed in U.S. Pat. No. 5,360,973.
Moderate carrier density increases the RF propagation loss property of the semiconductor. This RF loss can be used to absorb or block RF beams propagating through a spatial region with moderate carrier density. Very high carrier density increases the conductivity property of the semiconductor, until a “pseudometallic” state is reached. In a pseudometallic state, the semiconductor reflects incident RF beams. This reflectivity property can be used to block the RF beam in a transmission-type diffractive element, or can be used to reflect the RF beam in a reflection-type diffractive element. Webb, U.S. Pat. No. 6,621,459, teaches the use of high-intensity optical beams to generate pseudometallic carrier densities throughout the bulk of a thick semiconductor, which also serves as the phase delay layer.
Diffractive Efficiency Limits of Fresnel Zone Plates:
Prior art using wavelength-scale diffractive patterns (such as Fresnel Zone Plates and Photon Sieves) suffer from a fundamental limit on RF diffraction efficiency imposed by the coarse quantization of phase delay, which is structurally-determined by the number of physical RF phase delay layers in the modulator. The diffraction efficiency limit of such wavelength-scale Fresnel Zone Plate diffractive patterns is:
                    sin        2            ⁡              (                  φ          /          2                )                            (                  φ          /          2                )            2        .
The symbol phi is the phase quantization of the modulator in degrees. For example, in the simplest structure with only two RF phase delay levels separated by 180 degrees of phase delay, the maximum diffraction efficiency of 40.5% is given by the equation [Wiltse, 2003]. In a more complex modulator with three RF phase delay levels separated by 120 degrees of phase delay (0, 120, 240 degrees), the maximum diffraction efficiency is 68.4%. For a modulator with four RF phase delay levels separated by 90 degrees (0, 90, 180, 270 degrees), the maximum diffraction efficiency is 81%. Thus, to achieve moderate diffractive efficiency with a Fresnel Zone Plate or similar wavelength-scale diffractive pattern, many phase levels are needed, which requires a more-complex physical structure with more layers. It is also noted that this simple expression only gives the diffraction efficiency limit, and other additional efficiency penalties will result from attenuation losses, shadowing effects, and surface reflection losses. This diffractive efficiency limit will apply to any diffractive element designed according to the Fresnel Zone Plate equation.
Koolish, U.S. Pat. No. 6,720,936 B1, teaches the dynamic creation of reflective and absorptive regions to generate Fresnel Zone Plate diffractive patterns and the FZP-derived Photon Sieve diffractive patterns. Both of these structures use wavelength-scale diffractive features and are limited in maximum diffractive efficiency. Koolish teaches the use of programmable reflective surfaces including electronic paper, microelectromechanical systems (MEMs), and liquid crystal displays (LCDs).
Reits, U.S. Pat. No. 5,084,707, teaches the use of thin semiconductor layers (“plates”) with reflective zones formed by dense carriers patterned by laser, that are spaced with a phase delay material with a low loss coefficient and a dielectric constant nearly equal to 1.0, such as foams. These foams are fragile, and prone to mechanical or thermal damage. Foams are thermal insulators, and this insulation results in high temperature in the illuminated thin semiconductor plate. The low refractive index (dielectric constant) allows RF energy to enter and exit the foam with low surface reflection loss. However, the use of a spacer with low refractive index necessitates a physically-thicker device, and larger zone shadowing effects for off-axis RF beams. This reduces RF beamforming efficiency.
U.S. Pat. No. 5,360,973, teaches creation of Fresnel Zone Plates designed using Fresnel-Kirchhoff diffraction theory approximation. More specifically, photo-generated carrier plasmas are used to achieve MMW blocking (amplitude-modulating FZP). It also teaches the use of opposing-side electrodes that are “transparent” to MMW, but inject carriers into wafer that can be used as blocking pixels, with refresh times that are shorter than free carrier recombination times. Webb teaches the use of optically-transparent MMW back-plane reflectors using a fine metal mesh, a fine grid of conducting metal lines, or a coating such as Indium Tin Oxide (ITO). An optically transparent back-plane allows illumination from the back-side of the semiconductor.
U.S. Pat. No. 6,621,459 teaches the use of photo-carrier generation in semiconductors to achieve two modes of Fresnel Zone Plate operation, termed “improved blocking FZP” and “phase correcting FZP.” The “improved blocking FZP” is a method that uses lower light intensity to create an FZP, but with a penalty in the RF output level as a result of blocking-type operation. In the “phase correcting FZP” a higher RF output level is achieved, but with a penalty of much higher light intensity requirements. This patent teaches photo-generation of pseudometallic plasma density throughout the full thickness of a semiconductor layer with thickness on the order of the RF quarter-wavelength. Because of this, a very high illumination light intensity is required to operate a phase-correcting FZP